ACIDITY 295 



is one containing the molecular weight of the substance in grams 

 per liter. A molar solution of hydrochloric acid contains 36.5 

 grams (the molecular weight in grams) per liter. This concentra- 

 tion has 1 gram of hydrogen in a liter; it is, therefore, also a 

 normal solution. A molar solution of sulphuric acid contains 

 98 grams (the molecular weight in grams) of the acid in a liter. 

 This concentration has 2 grams of hydrogen per liter ; it is, there- 

 fore, twice normal. 



Molar concentrations of all compounds contain the same number 

 of molecules. 



It is still necessary to express concentration in percentage 

 when the exact constitution and therefore the molecular weight 

 of a substance are not known, as is true in the case of many 

 proteins. 



Hydrogen-ion Concentration. — Let us now consider the second 

 kind of acidity — that due to free hydrogen ions and not to 

 replaceable hydrogen atoms. Acid qualities have been ascribed 

 to one thing after another as science has advanced. Paracelsus 

 imagined the presence of an acidium primogenium. Lavoisier 

 thought oxygen to be the cause. And now hydrogen is known 

 to be responsible for acidity, either hydrogen so placed as to be 

 replaceable by a metal or hydrogen ions. From this latter 

 viewpoint, only substances capable of supplying hydrogen bearing 

 a positive electrical charge (H+ ions) are acids. 



Hydrochloric and acetic acids have certain properties in 

 common but to a different degree when viewed in the light of 

 their influence on physical, chemical, and physiological reactions. 

 Physically, hydrochloric acid conducts electricity better than 

 does a like (normal) concentration of acetic acid. Chemically, 

 hydrochloric acid is a much more efficient catalyst in the breaking 

 down of sucrose into glucose and levulose than is a like concentra- 

 tion of acetic acid. Physiologically, a normal solution of 

 hydrochloric acid is a poison, while normal acetic acid is a weak 

 artificial vinegar. Hydrochloric acid is, therefore, a stronger 

 acid than acetic. These acid qualities rest on the proportion of 

 free hydrogen ions present in the two cases. Only the hydrogen 

 atoms of the carboxyl groups (COOH) of acetic acid molecules 

 (CHs-COOH) — and of these, relatively few — dissociate; there- 

 fore, in a solution of acetic acid, there are mostly CHs-COOH 

 molecules and few CHs-COO" and H+ ions (the percentage of 



